A disk drive system is a data storage device. Among other things, a disk drive system includes a plurality of disks which are mounted for rotation about a common axis. Generally, each of the disks has a pair of disk surfaces which are coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. Data is stored digitally within concentric tracks on one or more of the disk surfaces.
Each disk surface has at least one magnetic transducer associated therewith. Generally, each of the magnetic transducers is attached on the end of an actuator arm. All of the actuator arms, and hence the transducers, are ganged together so that they move over their respective disk surfaces in unison. However, only one transducer is capable of reading data from or writing data to a disk surface at any given time.
During operation of a disk drive, each of the disks are rotated about an axis at a substantially constant rate. To read data from or write data to a disk surface, a magnetic transducer is positioned above a desired track of the disk while the disk is spinning.
Writing is performed by delivering a write signal having a variable current to the transducer while the transducer is held close to the desired track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetic polarity transitions into the desired track. The magnetic polarity transitions are representative of the data being stored.
Reading is performed by sensing the magnetic polarity transitions on a track with the transducer. As the disk spins below the transducer, the magnetic polarity transitions on the track present a varying magnetic field to the transducer. The transducer converts the varying magnetic field into an analog read signal that is then delivered to a read channel for appropriate processing. The read channel converts the analog read signal into a properly timed digital signal that can be further processed and then provided to a host computer system.
The transducer can include a single element, such as an inductive read/write element for use in both reading and writing, or it can include separate read and write elements. Transducers that include separate elements for reading and writing are known as "dual element heads" and usually include a magneto-resistive (MR) read element for performing the read function.
As is well known in the art, in order to properly position transducers over their respective disk surfaces, a disk drive system includes a servo system which uses servo information recorded on one or more of the disk surfaces. In general, there are two main types of servo systems. The first type of servo system is known as a dedicated servo system, where a single dedicated disk surface only includes servo information. The dedicated disk surface cooperates with a dedicated servo transducer, which is ganged together with data transducers, to provide positioning information to the data transducers so that the data transducers may be appropriately positioned over their respective disk surfaces. The second type of servo system is known as a sectored servo system and includes sectors of servo information which are interspersed between sectors of data on each of the disk surfaces. As a transducer flies over its respective disk surface it periodically obtains positioning information from the sectors of servo information recorded on the disk surface so that it can be properly positioned over the surface.
For the two types of servo systems described above, servo information is either written over an entire disk surface (as in the case of a dedicated servo system) or over periodic sections of a disk surface (as in the case of a sectored servo system). In either case, however, it is crucial that the servo information be written accurately.
Servo information is written during the manufacturing process. The process of writing servo information onto one or more of the disk surfaces is known as servo writing or servo track writing. In most conventional systems, an external device known as a servo track writer (STW), which includes its own transducer (STW transducer), is used to write a servo clock track onto a disk surface upon which servo information is to be written. The transducers of the disk drive system (as opposed to the STW transducer) are used to write servo information onto one or more of the disk surfaces.
More specifically, the transducers of the disk drive system are "placed" and "held" at an appropriate radial distance from the center of the disk using a variety of well-known techniques, such as by use of mechanical push-pin systems or optical push-pin systems (see description of mechanical push-pin systems and optical push-pin systems below). As the STW transducer reads timing information from the servo clock track, one of the transducers of the disk drive system is instructed to write servo information at a specified location (i.e., the position the transducer is being "held" at) on its respective disk surface based on the timing information read from the clock track. The transducer is then moved to a different radial location and the process is repeated. If servo information is to be written onto other disk surfaces, the above process is repeated with the transducers corresponding to the other disk surfaces. Accordingly, servo information is placed on one or more disk surfaces at specified radial distances and is based on the timing information read by the STW transducer from the servo clock track.
Of the well-known techniques used to "place" and "hold" transducers of a disk drive system at predetermined radial distances from the center of a disk, mechanical push-pin systems and optical push-pin systems have been most widely used. One type of mechanical push-pin system includes both a master arm (associated with an external device) having a master voice coil motor and an actuator arm (associated with the disk drive system) having a voice coil motor. The master arm and the actuator arm are mechanically linked by component known as a mechanical push-pin, wherein the actuator arm is biased towards the mechanical push-pin via its voice coil motor. By accurately positioning the master arm, the actuator arm may be accurately positioned at a predetermined location relative to the center of a disk. A transducer associated with the actuator arm is then used to write servo information onto a surface of the disk. The master arm is then repositioned to another predetermined location (the actuator arm moving with the master arm due to the mechanical link therebetween) and the actuator arm is then used to write additional servo information onto the disk surface.
Mechanical push-pin systems suffer from a number of significant drawbacks. For example, mechanical push-pin systems prevent disk drive systems from being sealed prior to servo writing due to their mechanical link. As is well-known to those skilled in the art, disk drive systems are commonly sealed prior to being shipped to an end user in order to prevent contaminants from interfering with the interface between a disk drive's transducers and the transducers' associated disk surfaces, among other reasons. Because mechanical push-pin systems require a physical link to be made between a master arm, which is external to the disk drive system, and an actuator arm of the disk drive system, the disk drive system cannot be sealed prior to servo writing. Accordingly, the servo writing process must be performed in a clean room environment, which may be costly to maintain and may add to manufacturing costs. Another drawback of mechanical push-pin systems is that they require the axis of the external master arm to be aligned with the axis of the actuator arm of the disk drive, due to the mechanical interaction between the push-pin and the actuator arm. Misalignment between the push-pin and actuator arm can cause the two to slip relative to one another resulting in mechanical vibrations. Accordingly, without proper alignment, the actuator arm will not properly write servo information at the appropriate radial distances from the center of the disk.
In an attempt to overcome some of the aforementioned problems with mechanical push-pin systems, some disk drive manufacturers have resorted to using optical push-pin systems, whereby the mechanical link between the master arm and the actuator arm is replaced by an optical link (hence the name optical push-pin). The optical link permits the disk drive system to be sealed prior to servo writing, which allows servo writing to be performed outside of a clean room environment. (This assumes, as will be understood by those skilled in the art, that the clock reader is of the non-contact variety.)
Optical push-pin systems generally include a master arm having both a light source and a sensor mounted thereon, a reflector mounted on the actuator arm of the disk drive system, a servo system for controlling the position of the master arm and a servo system for controlling the position of the actuator arm. Examples of optical push-pin systems can be found in International Publication No. WO 97/39450 entitled "Method and Apparatus for Non-Contact Servo Writing" and U.S. Pat. No. 5,486,923 entitled "Apparatus for Detecting Relative Movement wherein a Detecting Means is Positioned in the Region of Natural Interference."
One type of optical push-pin system includes a master drive assembly having both a master arm and a master voice coil motor, and a hard drive assembly having both a hard drive arm and a hard drive voice coil motor. The system also includes a first servo control system to accurately position the master arm and a second servo control system to position the hard drive arm relative to the master arm.
More specifically, the first servo control system includes a laser interferometer that detects the position of the master arm by monitoring light reflected off of a reflector mounted on the master arm. The detected position of the master arm is then compared to a desired position, which position is provided by an external source such as a computer. A servo compensation signal, which is based upon the difference between the detected and desired positions of the master arm, is then provided to the master voice coil motor to move the master arm to the desired position.
The second servo control system assists the hard drive arm in tracking the master arm. More specifically, the second servo control system includes a light source located on the master arm, a reflector mounted on the hard drive arm and a position sensor located on the master arm. The light source directs a beam at the reflector mounted on the hard drive arm. The beam is reflected and received by the position sensor located on the master arm. The reflector on the hard drive arm is designed so that the intensity of the reflected beam varies based upon where the beam, i.e., the one emanating from the light source, strikes the reflector. Movement of the reflector (and hence the hard drive arm) in one direction increases the intensity of the reflected beam, while movement of the reflector in the other direction decreases the intensity of the reflected beam. The detector senses the changes in intensity of the reflected beam and generates a servo compensation signal based upon the difference between the sensed intensity and a desired intensity. The servo compensation is then provided to the hard drive voice coil motor to move the hard drive arm to the desired position, i.e., so that it is aligned with the master arm. Once aligned, servo information may be written by transducers associated with the hard drive system. The master arm is then repositioned (the hard drive arm moving with the master arm due to the above-described optical link therebetween) and the process is repeated.
While the above-described optical push-pin system permits servo writing to be performed outside of a clean room environment, both it and systems like it have a number of significant drawbacks. For example, conventional optical push-pin systems require at least two servo loops which adds to the number of components required for the systems and, hence, their complexity and overall cost. Furthermore, since optical push-pin systems require movement of their light sources, targets and detectors, such systems may suffer from out of range errors if the target moves out of range of the light source and the detector. Moreover, because the actuator arm is designed to track the movement of the master arm, the rotational axes of the actuator arm and the master arm must be aligned with one another, which may be difficult in practice. Finally, such systems may suffer from positioning delays since a first servo loop is used to position the master arm and, only after the master arm is properly positioned, a second servo loop is used to position the actuator arm of the disk drive system.
Accordingly, there is a need to develop an apparatus which permits servo writing to be performed outside of a clean room environment and which measures the position of the actuator arm of a disk drive system directly, so that a single servo loop may be used during the servo writing process for positioning the actuator arm relative to a disk surface. In addition, it would be advantageous if the apparatus included a fixed light source which generates a beam that strikes the target throughout the range of motion of the target and a fixed detector which receives an interference pattern, reflective of the position of the target, throughout the range of motion of the target so that out of range errors are minimized or eliminated. The present invention is designed to overcome the aforementioned problems and meet the aforementioned, and other, needs.